The forked Gtk.CSS stylesheet with OS X window controls is designed specifically for elementary OS and its desktop environment: Pantheon.
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Installation
You'll need git to install this theme

Open terminal and execute the following
git clone https://github.com/surajmandalcell/elementary-x.git ~/.themes/elementary-x
Select this theme in tweak tool.
Thats it you're done!
Note: To install it globally i.e. for situation when you run apps as root move your app to /usr/share/themes